I enjoy these detours into the Mirror Universe and beyond. It gives authors a chance to let rip with their imaginations, and generally speaking, they deliver the goods. The stories presented here are like all Star Trek fiction: it's very much a case of "if you like that sort of thing". With that caveat established, I'll say the writing is of a high standard in the context of what you expect from Trek. The pattern here is that you get three distinct tales, each centred on the Mirror version of a character from the Trek universe. Naturally, they interact with other such characters.
My only criticism, and it's just a little niggle... the writers sometimes feel it's a cool idea to kill off a character. Take a well-established character, and obliterate his or her Mirror Universe counterparts. The trouble is, this becomes a bit tedious once the novelty wears off and, because you sometimes feel this is a throwaway idea, rather than central to the plot, it narrows the scope for sequels, and makes bookkeeping difficult, especially for multiple writers ("Oh, hang on, he died in so-and-so's story...")
Otherwise, it's a fun diversion, and the books in this series are nice and chunky, so a solid read is assured.